The present invention relates generally to precision liquid volume measurement devices and methods. More particularly, the invention relates to a device and method for se with a positive displacement metering device which can be expected to experience wear nd a resultant error during its operational life. The present invention is particularly uitable for gasoline dispensers.
Gasoline dispensers typically use positive displacement meters for precise volume measurement. These meters typically include various mechanical components which undergo wear during the life of the meter. This wear introduces an error from the meter's initial error-free calibration state. This error is referred to in the art as meter drift. The resulting loss of accuracy may cause a customer to be overcharged or a business to be credited improperly for the actual value of an individual transaction.
The meters are produced in volume to very exacting standards. Thus, each of a large number of meters can and does show a predictable wear pattern consistent with the others and will experience a similarly predictable error generation characteristic. The change in volumetric accuracy for a mass-produced positive displacement meter can and has been described as a function of the volume delivered over meter life.
Heretofore, this error has been addressed by periodic, manual meter calibrations using a calibration device called a "prover can." This type of calibration process requires human intervention to dispense a predetermined amount of liquid into the can and compare that amount to the meter's indication of the volume dispensed. A mechanical adjustment to the meter is then made to account for any error. Various weights and measures authorities require performance of this type test at regular intervals, usually on an annual basis. This type of meter calibration process is expensive and labor-intensive.
Previous attempts to incorporate a "prover can" type device into a gasoline dispenser housing are not satisfactory. An example is the system shown in U.S. Pat. No. 4,831,866 to Forkert et al. The system shown in that patent does not take advantage of the known body of information concerning meter wear and the resultant error.
Therefore, there is a need for a simple, low cost system which takes advantage of known positive displacement meter error to continuously correct meter output and compensate for a known and expected error rate.